Johnson was born Monica Lenore Belson in 1946 in Colorado, but was raised in El Centro, California.[1] Her brother, Jerry Belson, a longtime Emmy Award winning screenwriter and film producer, hired her to type scripts for the TV series The Odd Couple circa 1972; noticing that his sister added jokes to the scripts which met with the producers' approval, he suggested that she partner with Marilyn Suzanne Miller to form a writing team. Initially working under her married name of Monica Mcgowan in 1973, she and Miller penned three scripts for The Mary Tyler Moore Show. For the second script, having remarried, she was credited as Monica Mcgowan Johnson. By the time of third script in 1974, she was credited simply as Monica Johnson, the professional name she would use for the rest of her career.

Miller and Johnson broke up as a writing team in 1974; Miller would go on to become one of the original writers for Saturday Night Live starting in 1975.

Johnson later became a writer/producer on Laverne & Shirley, and worked on a couple of TV movies before beginning her long-term screenwriting collaboration with Albert Brooks in 1979 with the film Real Life. The two would co-write 5 more of Brooks' films over the following two decades.